1. Field of the Invention
Embodiments of the invention generally relate to an apparatus and method for conducting chemical analysis of substrate plating solutions.
2. Description of the Related Art
Metallization of sub-quarter micron sized features is a foundational technology for present and future generations of integrated circuit manufacturing processes. More particularly, in devices such as ultra large scale integration-type of devices, i.e., devices having integrated circuits with more than a million logic gates, the multilevel interconnects that lie at the heart of these devices are generally formed by filling high aspect ratio interconnect features with a conductive material, such as copper or aluminum, for example. Conventionally, deposition techniques such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) have been used to fill interconnect features. However, as interconnect sizes decrease and aspect ratios increase, efficient void-free interconnect feature fill by conventional deposition techniques becomes increasingly difficult. As a result thereof, plating techniques, such as electrochemical plating (ECP) and electroless plating, for example, have emerged as viable processes for filling sub-quarter micron sized high aspect ratio interconnect features in integrated circuit manufacturing processes.
In an ECP process, for example, sub-quarter micron sized high aspect ratio features formed into the surface of a substrate may be efficiently filled with a conductive material, such as copper. ECP plating processes are generally two stage processes, wherein a seed layer is first formed over the surface and features of the substrate, and then the surface and features of the substrate are exposed to a plating solution, while an electrical bias is simultaneously applied between the substrate and an anode positioned within the plating solution. The plating solution is generally rich in ions to be plated onto the surface of the substrate, and therefore, the application of the electrical bias causes these ions to be reduced and thereby plated onto the seed layer. Furthermore, the plating solution generally contains organic additives, such as, for example, levelers, suppressors, and accelerators configured to control the plating distribution throughout the plating process. These additives are generally maintained within narrow tolerances, so that the repeatability of the plating operation may be maintained.
Conventional ECP systems generally utilize a cyclic voltammetric stripping (CVS) process to determine the organic additive concentrations in the plating solution. More particularly, three electrodes, a working electrode, a counter electrode, and a reference electrode, are immersed in a cell having a plating solution to be measured therein. The reference electrode and the working electrode are typically connected to a device for measuring the electrical potential difference between the respective electrodes. The reference electrode generally consists of three components, a half-cell electrode, a half-cell electrolyte, and a reference junction. As used herein, the term “half-cell electrode” generally refers to a solid phase, electron-conducting contact within the half-cell electrolyte, at which contact a half-cell oxidation-reduction reaction occurs that establishes a stable potential between the half-cell electrolyte and the working electrode. Direct physical, and therefore electrical contact between the half-cell electrolyte and the sample plating solution is established through the reference junction, which usually consists of a porous ceramic, glass, or plastic plug (e.g. frit), or other device capable of achieving a fluid mechanical leak having pores large enough to allow equal transport of anions and cations. The reference junction is necessary to establish electrical contact with the plating solution, and therefore, the working electrode. Conventionally, the potential of the working electrode is swept through a voltammetric cycle that includes both a metal plating range and a metal stripping range. The potential of the working electrode is swept through at least two reference baths of non-plating quality, and an additional bath where the quality or concentration of organic additives therein is unknown. In this process, an integrated or peak current used during the metal stripping range may be correlated with the quality of the non-plating bath. As such, the integrated or peak current may be compared to the correlation of the non-plating bath, and the quality of the unknown plating bath determined therefrom. The amount of metal deposited during the metal plating cycle and then re-dissolved into the plating bath during the metal stripping cycle generally correlates to the concentration of particular organics in the plating solution. CVS methods generally observe the total copper ions reduced on an electrode over a predetermined potential range. Inasmuch as accelerators or brighteners counteract the suppressors to increase the plating rate, their quantities may be determined from observation using standard addition or dilution titration techniques.
Generally, measured quantities of additives are injected from the top of the cell into the plating solution using syringes or tubes for testing the plating solution. Unfortunately, as/test volumes may vary from a few milliliters to several hundred milliliters, the cell size must be changed accordingly to accommodate the differing test volumes. Further, as tubes or syringes are used to inject the additives into the plating solutions, it is difficult to accurately inject a microliter or less of the additives into the plating solutions as the volume of the additives must be large enough to be dispensed as a droplet. Micro amounts of additives may be injected by immersing the tube tips into the plating solution. However, residual additives contained within the tubes may diffuse out into the reference bath during the test and contaminate the measurement. Accordingly, due to the potential variation of additives due to the imprecise injections, a plating solution under test may be incorrectly analyzed and therefore cause a plating problem that may affect several batches of substrates affecting the plating throughput, and may ultimately increase the cost of production.
As such, there is a need for an efficient and cost effective apparatus and method for plating solution analysis.